The present invention relates to a novel solution treatment method by which the disappearance of the sensitized structure produced at part of the steel pipe, wire stock, plate stock, or material of arbitrarily-shaped cross section (hereinafter called "stainless material" for the sake of simplicity) which is formed of an austenitic alloy such as austenitic stainless steel by the thermal effect of heating or working accompanied by a rise in temperature at the time of subjecting partially said stainless material to said heating or said working accompanied by a rise in temperature, for example, welding or bending, can be achieved without newly forming the sensitized structure. Stainless material is excellent in corrosion resistance and mechanical properties, and stainless steel pipes, etc., are now widely used in a wide variety of equipment and installations such as nuclear reactors and chemical plants in a corrosive environment.
In building the above-mentioned nuclear reactor equipment or chemical plant, stainless material is often assembled into a part or incorporated into the mainbody of equipment in a factory or in a job site while being subjected to heating and working accompanied by a rise in temperature (such as hot bending or welding) at part of said stainless material. It is a phenomenon that carbide often precipitates on the grain boundaries within the structure of thermal boundaries between the places which have been subjected to said heating or working accompanied by a rise in temperature and the places which have not been subjected to said workings. This sensitized phenomenon results when the structure of the material, i.e. the material existing between the places which have been subjected to heating or working accompanied by a rise in temperature and the places which have not been subjected to said workings, is put in the range of temperatures of about 400.degree. C. to about 800.degree. C. (the socalled "a sensitizing temperature region") in the course of partially subjecting stainless material to heating or working accompanied by a rise in temperature. It is known that when liquid, gas, etc., containing corrosive substances, for example, such halogen ions as Cl.sup.-, etc., or dissolved oxygen come into contact with the stainless material of which structure has been thus partially sensitized, the stainless material will corrode electrochemically.
In addition, it is known that when partially subjecting the stainless material to the above-described heating or working accompanied by a rise in temperature, tensile stresses may remain in the material and the residual stresses may act on the abovestated electrochemically-corroded portions to cause stress-corrosion cracking.
For example, in a case where the above-described stainless material is a pipe, the liquid passing into the pipe after the pipe is incorporated into the mainbody of a nuclear reactor equipment by welding may contain corrosive substances. The stainless steel pipe is electrochemically corroded by the corrosive substances at the portions of sensitized structure formed in the neighborhood of weld of the pipe and, in addition, the residual tensile stresses produced by welding act on the corroded portions, so that said pipe is sometimes cracked by stress-corrosion, giving rise to a grave situation from the view point of operation or safety of the equipment.
That stainless material which has been subjected not only to the above-stated welding but also to hot bending operation or other workings including local heating work and working accompanied by a rise in temperature suffers such corrosion or stress-corrosion cracking is well known and strenuous efforts have been made to work out a countermeasure.
There are indications that the stainless material will profit from being subjected to solution-treatment (also called "solid-solution treatment") at the part of sensitized portions to avoid corrosion or stress-corrosion cracking at the portions which have been subjected to local heating or working accompanied by a rise in temperature. However, the ordinary method is that of raising the temperature of an article to be treated which has been locally sensitized to a temperature (approximately 1090.degree. C., hereinafter the same temperature) at which carbide will enter into solid solution in a heating furnace followed by cooling the article, and since even the portions requiring no treatment will come to be subjected to the solution-treatment, it follows that this method is unreasonable and poor economy. In addition, it may be impossible for the large sized parts of the stainless material to be subjected, as a whole, to such a solution treatment as that described above.
Accordingly, as for the large sized parts of the stainless material, it has been a practice to subject only the portions which have been locally sensitized by a thermal effect during the preceding operation to the solution treatment in such an equipment as the above-mentioned heating furnace. However, when being subjected to the partial solution treatment in such a manner, the sensitized structure which has been produced during the preceding operation is transformed to another state in which carbide will enter into solid solution, but a new sensitized structure is produced at the thermal boundaries between the portions heated and the portions not-heated during the second solution treatment. In other words, this manner suffers the defect that the desired object cannot be eventually accomplished.
Additionally, for example, on giving consideration to the corroded or stress-corrosion cracked portions of the parts made of the stainless material and used in nuclear reactor equipment, even where the parts have been previously subjected to solution treatment, no effective method has been found since the sensitized structure is formed at thermal boundaries during the welding operation or high-temperature brazing operation in the assembling of said parts or in the incorporation of said parts into the main body of the equipment at the job site. Therefore, it is necessary to use the parts as sensitized and this forms the main cause of occurrence of the corrosion or stress-corrosion cracking.
The present invention has been made with the object of providing a method for localized treating of stainless material sensitized as stated above at the thermal boundaries of the stainless material during the incorporation of said material into the main body of such an equipment as nuclear reactor equipment, chemical plant, or other equipment by subjecting the material to a working such as local hot-bending or welding operation in which the temperature of part of said material is raised to a high temperature by heating in a factory or in a job site in the fabrication of the nuclear reactor equipment, chemical plant, or other equipment. In the present invention, the carbide will enter into solid solution without exercising an ill effect upon other sound structure portions to such a degree that it may safely said that there is no said effect at all even in a case where said material is an independent part or even after said material has been incorporated into the main body.
The constitution of the present invention is chiefly characterized by rapidly heating the material of which structure has been partially sensitized, as in the neighborhood of weld of said material, by heating or working accompanied by a rise in temperature, either at the sensitized structure portions alone or at neighboring portions to a temperature at which carbide will enter solid solution, followed by rapidly quenching the material within a treating time allowed according to the carbon content of said material.